Many applications in fields such as photolithography and medicine require a laser operating at specific wavelengths, such as wavelengths of 308 nm using XeCl, 248 nm using KrF, 193 nm using ArF, and 157 nm using F2. These applications typically require low energy, such as on the order of a few tens of millijoules, and a relatively high repetition rate of operation, such as on the order of hundreds to several thousands of pulses per second. These applications also require operation with very high reliability and low operating cost. There are several difficulties involved in obtaining a stable discharge in these lasers, due in part to the operating voltage required as a result of the low energy and small beam size, which are not present in lasers of higher energy. This stable discharge is necessary for the laser gas to have a sufficiently long lifetime. These performance requirements impose difficult constraints on the design of a laser pulser.
Such a laser is typically direct discharge pumped, normally at voltages in the range of 30 kV and at pulse repetition rates above 1 kHz. Peak electrical power input to the laser can be several tens of megawatts. Furthermore, to make the lithographic process commercially viable the equipment must not exhibit unscheduled down time and must deliver pulses of the highest stability, uniformity, and spectral quality for uninterrupted periods of weeks at a time. These requirements have in recent times led to the development of pulsers driven by solid state switches as an improvement on switch life. Replacement of the gaseous thyratron with a solid state switch has been proven to greatly extend laser service intervals and hence reduce operating costs, but whereas the thyratron operating range covers voltages of 20–30 kV, best utilization of solid state switch capabilities occurs at lower voltages, typically in the range of 1–5 kV.
A solid state switch can be used to drive a step-up pulse transformer and a multi-stage pulse compressor to reach correct laser operating voltage and voltage risetime. The attainment of the necessary voltage level, in the range of 30 kV, with sufficiently low circuit inductance, in the range of tens of nH or less, at multi-kilowatt average power levels is typically done with transformer oil, vapor phase coolants, or pressurized gas such as sulfur hexafluoride or nitrogen. Atmospheric air does not possess sufficient dielectric strength to withstand the necessary voltage stress or the necessary thermal properties to dissipate the generated heat. Leak-free containment of oil over long time periods is known to be difficult. Vapor phase coolants are expensive and primarily suited for heat removal rather than voltage insulation. Gas containment at the necessary several atmospheres pressure requires use of thick-walled pressure vessels and elaborate seals. In addition, for the above approaches a heat exchanger and pump are required to extract heat from the cooling medium. Using solid dielectrics such as thermal compounds in paste form in present pulser designs is cost prohibitive and would generate excessive temperature gradients due to their basic thermal properties. The low voltage portion of such a pulser operates at high effective currents that require cooling, and the high voltage portion requires positive air displacement to prevent corona generation and resulting breakdown. These requirements exist due in part to the high voltages, currents, and rates of change of these voltages and currents and the dimensional constraints imposed by the geometry of the laser system.